Issue 3687 (part 1): Provide a compress MM function for MM rests only

Issue 3687 (part 1): Provide a compress MM function for MM rests only

  Multi-measure rests were previously compressed (i.e. all bars
  following the first were removed) by using the \compressFullBarRests
  predef.  This simply turned on 'skipBars, which causes bar lines at
  the ends of bars which contain either no rhythmic event or contain
  a rhythmic event with a duration extending beyond the bar to be removed.
  Such bars are generated by notes with durations longer than a bar,
  like a1*3, or by shorter notes which extend beyond the bar.

  This patch provides a function which also uses 'skipBars, but with
  a filter so that 'skipBars is activated only when an actual MM rest
  is encountered and deactivated by the first following rhythmic event.